(a) Field of the Invention
The present invention relates to a method and apparatus for inspecting the quality of laser welding, and more particularly, a method and apparatus for inspecting the quality of laser welding by monitoring the size of a metal molten pool (i.e., weld metal) during a laser welding process.
(b) Background Art
In general, laser welding is a joining technique used to join metals or non-metals in order to melt and fuse the metals or non-metals together using a laser beam emitted from a laser as a heat source as shown in FIG. 1.
When a laser beam is irradiated onto base metals, a keyhole is created by the laser beam and a weld metal surrounding the keyhole is molten. Then, the resultantly formed keyhole and the molten metal pool are moved continuously in a particular direction, i.e., a welding progress direction of the base metal in order to perform the welding process (e.g., longitudinal).
The laser welding used as a vehicle body assembly technique is a method that processes materials using a densely focused laser beam. Such a laser welding has an advantage in that thermal deformation is small, productivity is high, and the materials to be processed are less restricted, but requires a relatively precise weld matching operation as compared to spot welding.
For such a laser welding, a defect of the welding quality occurs as follows.
In case of galvanized steel plates, a zinc vapor discharge gap in the order of 0.1-0.2 mm is required to inhibit the cracking of the weld bead.
If a gap between welding base metals is less than approximately 0.1 mm, there occurs a welding failure such as the cracking of the weld bead as shown in FIG. 1(a). On the other hand, if the gap between the welding base metals exceeds approximately 0.2 mm, there occurs a welding failure such as undercuts as shown in FIG. 1(b).
A conventional laser welding quality inspection method based on the monitoring of a laser processed light will be described hereinafter.
FIG. 2 is a schematic view illustrating a conventional laser welding quality inspection system based on the monitoring of a laser processed light, and FIG. 3 is a schematic view illustrating a wavelength band of a laser welding processed light (or light).
As shown in FIGS. 2 and 3, a welding processed light emitted from a welding part during a laser welding process is converted into an electrical signal, and a signal on which determination of good welding quality is based is set as a reference waveform. An upper limit value and a lower limit value are decided based on the reference waveform so that when a detected signal is beyond the region of the upper and lower limit waveforms, the welding part is determined to have a welding failure
A laser welding processed light typically includes a near infrared ray (NIR), a near ultraviolet ray (NUVR), a laser reflected light, and the like. Generally, the laser reflected light occupies the majority of the laser welding processed light.
A conventional technology employs a method in which a laser processed light is converted into an electrical signal and the converted electrical signal is simply compared with a reference waveform, followed by analysis. However, since the laser reflected light occupies the majority of the laser welding processed light, a signal associated with the length of a defective weld, the width of a weld bead, or the like is small, which makes it impossible to correctly measure the quality of the laser welding. For this reason, such a signal is mainly used for monitoring a laser power. Besides, since a trend of the laser power monitoring signal is analyzed and set using a statistical method of collecting data for about two weeks, a professional technique that makes it very difficult for a general worker to set the signal is required. In addition, there is involved a problem in that a technician of an automobile maker mostly sets the signal personally, and hence a lot of time and cost is spent.
In other words, the laser welding processed light includes a near infrared ray (NIR), a near ultraviolet ray (NUVR), a laser reflected light, and the like. Generally, since the laser reflected light occupies the majority of the laser welding processed light, the laser welding quality cannot be correctly investigated by the conventional method in which an electrical signal of the sensed welding processed light is simply compared with a reference waveform, followed by analysis.
Thus, as shown in FIG. 4, primarily, it is required that a laser reflected light should be optically filtered and removed from a sensed laser welding processed light. The wavelength band of a laser processed light that has passed through a chromatic aberration filter is a range of the near ultraviolet ray (NUVR) and the near infrared ray (NIR), which is a plasma light generated from a laser welding molten pool.
In FIG. 5, there is shown an electrical signal of the sensed plasma light.
The electrical signal of the plasma light is divided into an AC component indicating an irregular white curve portion in a graph of FIG. 5 and a DC component pulsating instantaneously. In this case, the plasma is closely related with the state of a molten pool and the welding quality.
As a result of analysis of a photographed molten pool of a laser welding part using a high-speed camera capable of taking more than 500 frames-per-second (fps), it can be seen that a series of numerous processes are repeatedly performed in which the laser welding part absorbs a laser beam condensed thereto to cause a solid-state metal thereof to be melt, and an optic head is moved in a welding progress direction so that a rear portion positioned 1-3 mm away from a focus of the laser beam is rapidly coagulated to produce a weld bead.
The state of the molten pool of the laser welding part in the same base metal varies susceptibly depending on a gap between a base metal panel and a base metal panel, which it is desired to weld.
If this gap is large, the molten pool melt by the laser beam is mostly filled in the gap, and thus there occurs a welding failure such as an undercut in which an upper portion of a produced weld bead is sunken.
In addition, since the size of the upper portion of the molten pool becomes small, the strength and the instantaneous variation of a plasma light generated externally from the welding part are decreased.
On the other hand, if the gap is proper, since the amount of the molten metal pool filled in the gap is small, and thus the size of the upper portion of the molten poll becomes large. In addition, the strength and the instantaneous variation of a plasma light generated externally from the welding part are increased, and the waveform of the plasma light is also very stable.
As discussed above, in a multi-fold overlapping welding of a galvanized steel plate, if a gap between both panels is less than 0.1 mm, zinc vapor entrapped in the metal molten pool is discharged to the outside through the weld bead into plasma, and thus the strength and the instantaneous variation of a plasma light is further increased as compared to the case of a proper gap (good in welding quality), thereby making the waveform of the plasma light unstable.
Accordingly, methods are proposed in which an electrical signal of this plasma light is monitored to inspect the quality of the laser welding.
However, in the same laser welding part of a vehicle body, since a variation of the DC component of the plasma signal varies significantly depending on the state and the matching relation of the panel, it is difficult to correctly check the quality of the laser welding using the variation of the DC component of the plasma signal.
Thus, a technology has been recently proposed which performs the quality inspection of the laser welding by partially employing an AC component from which the DC component of the plasma signal is filtered. For example, such a technology is exemplified by Japanese Patent Nos. P2000-271768A and P2001-48756.
However, since the above-mentioned Japanese Patent documents employ a simple filtering method which removes only a DC component of less than a specific frequency, a welding processed light signal of other wavelength bands should be additionally sensed and should be subjected to a significantly complicated process to implement a laser welding quality inspection method due to a reduced discriminating power between an AC component of a welded part which is good in welding quality and a sensor electrical signal of a welded part in which a welding failure occurs.
For example, the above-mentioned Japanese Patent documents perform the inspection of the quality of the laser welding using a total of eight sensor signals including two sensor signals, a signal indicative of a laser reflected light, a signal indicative of a plasma light, a signal of a DC component, and a signal of an AC component.
In addition, collecting data of a welded zone where welding is good and a welded zone where welding is defective should occur through numerous tests.
Actually, innumerable combinations of the laser welding base metals exist in a vehicle body laser welding line. Thus, in case of using the above method, it is in fact impossible to secure the data with respect to the entire laser welding part of a vehicle body in terms of efficiency of welding time arrangement, which makes it impossible to apply to a vehicle body welding process.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known to a person skilled in that art.